As examples of a conventional light source to be installed in a fluorescence microscope, may be mentioned discharge lamps such as a short-arc ultrahigh pressure mercury lamp, a metal halide lamp and a xenon lamp. In addition, from the viewpoint of energy saving, downsizing of a device, increasing life of a light source and the like in recent years, using a solid light-emitting element such as a light-emitting diode as a light source has been proposed (see Patent Literature 1).
When solid light-emitting elements are used as light sources, however, a solid light-emitting element that emits light with a wavelength of 500 to 550 nm, out of three types of solid light-emitting elements that emit red, green and blue light, cannot provide high luminance at present. Thus, it is difficult to obtain sufficient brightness in the field of view of the fluorescence microscope.
In view of such circumstances, a light source apparatus including an excitation light source such as a laser diode and a phosphor that emits fluorescence with a wavelength of 450 to 650 nm upon the receipt of excitation light (for example, light with a wavelength of 440 to 470 nm) from the excitation light source is employed in place of the solid light-emitting element that emits light with a wavelength of 500 to 550 nm.
Moreover, the fluorescence microscope is provided with a bandpass filter that selectively transmits light in a wavelength region of 460 to 550 nm, for example, in order to irradiate a fluorescent dye used for observing a sample only with light in the excitation wavelength region of such a fluorescent dye.
However, it has been found out that problems as described below arise when such a light source apparatus is employed as a light source apparatus installed in a fluorescence microscope.
A fluorescent dye having an excitation wavelength region of 450 to 600 nm and a fluorescence wavelength region of 500 to 650 nm is widely employed as the fluorescent dye used for observing a sample in the fluorescence microscope. On the other hand, Ce:LuAG, for example, is known as the phosphor that emits fluorescence with a wavelength of 450 to 650 nm. The fluorescence wavelength region of such Ce:LuAG is a wavelength region of 470 to 620 nm. When the fluorescence spectrum of the phosphor is shifted to a longer wavelength side than the excitation spectrum of the fluorescent dye as just described, a spectrum represented by the product of the spectrum of light irradiated onto the fluorescent dye and the excitation spectrum of the fluorescent dye (hereinafter referred to as a “sensitive spectrum”) is shifted to a longer wavelength side than the excitation spectrum of the fluorescent dye. Consequently, a wavelength region in which the sensitive spectrum and the fluorescence spectrum of the fluorescent dye overlap with each other is increased, thus disadvantageously increasing background noise when a sample is observed.